I am running a bitcoin node on my virtual private server. It costs me $5/month and I use it for many things.

The nodes do not create the block chain, they just validate it. It is the miners that extend the block chain. To corrupt the block chain you would need the majority of the mining power and the majority of the nodes. If you have just the mining, you can win the races to create the blocks, but your blocks would be rejected by the majority of the nodes and so would not propagate. If you just have the nodes, you cannot create the bogus blocks. I believe you need both.

So to corrupt the bitcoin block chain you need the majority of the mining power, and since mining requires a lot of electrical power, you would need to dedicate a tremendous amount of electricity to the project. I did a quick search and found one source claiming that bitcoin mining consumed 67TWh in 2015. The corresponds to 7.5GW dedicated full time to bitcoin mining. I usually equate a nuclear reactor to about 1GW, so that is the equivalent to 7.5 nuclear reactors worth of power. That would only be within the reach of the most energy rich nation states such as the US, China or Russia, but they would have to be very highly motivated.

I just found http://www.cbeci.org (you have to ignore the certificate errors), which is a University of Cambridge website that estimates current mining energy usage, it is estimating 9GW with bounds from 3.6GW to 17GW.

Presumably once bitcoin becomes a reserve currency, the nation states themselves will have a strong incentive to defend the block chain and they will set up the means to bring additional mining online if it looked like there was a risk of a 51% attack.